The Green party is rethinking its approach to electoral alliances after losing half of its votes in the general election when it stood aside for a string of Labour and Liberal Democrat candidates to help prevent Theresa May from winning a majority.

Caroline Lucas, the Green party co-leader, defended the strategy of working with other parties on the left but said more could be done in future to make sure Green candidates only stood aside in seats where the Labour or Liberal Democrat candidate supports voting reform and the seat was marginal enough to have a chance of beating the Conservatives.

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The issue caused a split at the party’s autumn conference in Harrogate, North Yorkshire, where delegates voted to acknowledge that the strategy had successfully mobilised anti-Tory supporters but caused a “negative impact on overall vote share”.

Lucas and her co-leader, Jonathan Bartley, praised the party’s performance in the 8 June poll, arguing it was the second best since the party began, and stressed it was up to local parties whether they chose to fight seats.

But Lucas suggested the party would do some things differently next time. She told the Guardian: “The electoral alliance strategy we passed at the last conference was basically a three-year strategy. But it had to be concertina-ed into six weeks. And the amount of time we had to do political education in the party and really go through which seats were marginal seats – that I think we would look at again.

“What happened in some of the seats is that we had people standing down in seats where there was no strategic need to do it. That is something everyone is acknowledging we need to get sharper about. That bit would, I think, be done differently.”

In its analysis of the 2017 election, the party’s political committee said the Greens should aim to “stand in as many places as possible and maximise vote share with a distinctive narrative” in future snap elections.

The party gained 1.6% of the vote in June, compared with 3.8% in 2015, and made no inroads in target seats such as Bristol West and Norwich South, where the incumbent Labour MPs increased their majorities.

Bartley said the strategy had worked better in some areas than others. “In some areas, people felt quite bruised by it and some areas they came out very positively. And the political terrain has changed within those 22 seats,” he said.

In his speech to about 300 members in the conference auditorium, he said the decision for candidates to stand aside had been brave. “If the other parties had been willing to work with us, Theresa May wouldn’t be sitting in No 10 today,” he said.

“It hurt. And I know many of us still feel that hurt. I do. But let’s not forget what we achieved together. Our ideas and policies are now common currency. Part of the mainstream. We achieved the party’s second best general election result. And we helped deny Theresa May her majority and her mandate.”

Afterwards, delegates spoke of how the experience of Green candidates having to stand aside had caused bitter splits in the party. Klina Jordan, a member from Bristol West, said there had been “intemperate language on all sides of the debate over the summer”. Some people who opposed electoral cooperation had accused those in favour of “betraying the party and being traitors”; some on the other side had hit back, saying its opponents were “naive and have no political nous”.

An amendment was passed at the conference in favour of acknowledging that electoral alliances had positive and negative impacts, but a motion calling for a new test to make sure all future pacts were in the best interests of the party was rejected.

The debate over how to update the party’s policy on electoral alliances will continue on Tuesday, and there will be a further session on whether to ask the party leadership to stop referring to the Greens as part of a “progressive alliance” when Labour and the Lib Dems are not reciprocating.

The party is also focused on how to win back younger supporters attracted by Labour at the election, which took an estimated three-quarters of the voters who deserted the Greens. There is now a debate within the Greens about whether to focus more on market towns and less on cities such as Bristol and Norwich where students voted for Jeremy Corbyn’s party in their droves.

Lucas said the party was reviewing its target seats and that she thought many young people had not left the Greens in their hearts but had made a tactical decision to support Labour this time.

“To talk about how we get those people back, I think focusing on the environment and Brexit will bring them back. Corbyn’s position on Brexit I think betrays young people,” she said.

The Brighton MP, who left the main conference speech and round of media interviews to Bartley, said it was an “impossible question” whether she would co-lead the Greens into the next election but she believed the jobshare was “working brilliantly”.

• This article was amended on 10 October 2017 to expand upon what Lucas meant about the future strategy of the Green party in relation to standing aside in marginal seats, specifically the importance of only standing aside where the Labour or Liberal Democrat candidate supports voting reform.